Linear Compressor And Corresponding Drive Unit

ABSTRACT

A drive unit for a linear compressor comprising a frame and an oscillating body. The oscillating body is mounted in the frame via at least one diaphragm spring and can be moved back and forth in one direction. The diaphragm spring comprises a plurality of limbs, fastened with one end to the frame and with the other end to the oscillating body. Two limbs each have inversely curved sections between the two ends.

This invention relates to a linear compressor, in particular for use forcompressing refrigerants in a refrigerating device, and in particular adrive unit for driving an oscillating linear piston movement for such alinear compressor.

U.S. Pat. No. 6,506,032 B2 discloses a linear compressor whose driveunit comprises a frame and an oscillating body mounted in the frame bymeans of one diaphragm spring. The oscillating body comprises apermanent magnet, a piston rod rigidly connected to the permanent magnetand a piston articulated to the piston rod, which piston can be movedback and forth in a cylinder. The movement of the piston is driven by anelectromagnet arranged around the cylinder, which electromagnetinteracts with the permanent magnet. A disc-shaped diaphragm spring isscrewed centrally to the piston rod, and the outer edge of the diaphragmspring is connected to a yoke which surrounds the cylinder, theelectromagnet and the permanent magnet.

The oscillating body and the diaphragm spring form an oscillating systemwhose natural frequency is determined by the mass of the oscillatingbody and the diaphragm spring, as well as by the stiffness of thediaphragm spring. The diagram spring only permits small oscillationamplitudes because any deflection of the oscillating body is associatedwith an expansion of the diaphragm spring. Due to the low oscillatingamplitude it is difficult to reduce the dead volume of the cylinderreliably. However, the higher the dead volume the lower the efficiencyof the compressor. The short stroke also necessitates designing thecylinder with a diameter that is proportional to the length in order toachieve a given throughput. It is expensive to seal the correspondinglylarge circumference of the piston sufficiently.

Since the oscillating body is only retained in the radial direction byits connection to the spring, it is possible that the head of the pistonrod supporting the piston may oscillate back and forth and grind againstthe cylinder wall. To prevent this a compressed gas bearing is providedfor the piston, i.e. the cylinder wall covered by the piston hasopenings which are connected to the high pressure outlet of the linearcompressor to form a gas cushion between the inner wall of the cylinderand the piston. However, such a compressed gas bearing only functions ifthe required excess pressure is present at the outlet of the linearcompressor, i.e. not when the compressor starts or stops. At these timesthere is a risk that the piston will grind against the cylinder wall,resulting in premature wear of the compressor.

A linear compressor is disclosed in U.S. Pat. No. 6,641,377 B2. In thisdouble-piston linear compressor each piston is retained by two two-armeddiaphragm springs.

Due to the curvature of the limbs a longer piston stroke is possible,but each diaphragm spring exerts a torque on the piston when deflected.If this torque is not exactly compensated for, the piston performs arotary oscillation in addition to its linear oscillating movement, andwobble movements of the piston may be excited which may result incontact between the piston and cylinder and consequently to increasedwear.

The object of this invention is to provide a low-wear drive unit for alinear compressor with a frame and an oscillating body mounted by meansof a diaphragm spring, in which the diaphragm spring permits a longstroke of the oscillating body and which is able to achieve a highthroughput with a small piston diameter.

The object is achieved in that the plurality of limbs of the diaphragmspring each engage with one end on the frame and with the other end onthe oscillating body, and in that the limbs have pairs of sections withopposing curvature between the two ends. The limbs do not thereforeextend along the shortest path between the two ends, so that when theoscillating body is deflected, they stretch and my approach therectilinear shape without the material of the limbs having to beexpanded for this purpose. Within the same workpiece it is very easy toproduce the limbs so that their torques exactly compensate each other;if, as described in U.S. Pat. No. 6,641,377 B2, two diaphragm springsare provided with limbs covered in different directions, deviations inmaterial strength from one spring to another may prevent suchcompensation or at least render it extremely difficult.

The diaphragm spring preferably has pairs of limbs with sections curvedin opposite directions.

In the simplest case each limb has an individual section curved in onedirection. Each such limb also exerts a torque on the oscillating bodysupported by it when deflected, but this is compensated for by the limbpaired with it and curved in the opposite direction.

Each limb preferably has two sections curved in different directions.Since the differently curved sections also generate torques in oppositedirections in this case too, the torque of each individual limb maytherefore be made very small or caused to disappear altogether.

It is also advantageous to provide at least a second diaphragm springwhose limbs engage on a region of the oscillating body which is distantfrom the region of engagement of the first diaphragm spring in thedirection of the oscillating movement. The oscillating body is reliablyguided linearly in the direction of the desired oscillating movement bythe two diaphragm springs, and a lateral deflection movement, whichcould result in contact between a piston supported by the oscillatingbody and a cylinder surrounding the piston, can be avoided.

The limbs of the same diaphragm spring are preferably joined integrallytogether at their ends engaging on the frame and/or at their endsengaging on the oscillating body. The ends engaging on the frame mayalso be connected by a frame integral with the leaf springs.

To provide a long stroke without risk of material fatigue, the limbs ofthe at least one diaphragm spring should be produced from a very thinmaterial. Its strength may be dimensioned so small that it is onlysufficient to prevent lateral deflection of the oscillating body.However, such a weak diaphragm spring would result in a low naturalfrequency of the drive unit and hence, at a predetermined stroke, in alow throughput of a compressor driven by the drive unit. To achieve anatural frequency of the drive unit sufficient for the requiredthroughput, each limb is preferably assigned a readjusting spring whichcounteracts deformation of the limb so that the diaphragm spring,together with the readjusting springs, form an elastic system whosestiffness is considerably higher than that of the diaphragm springalone.

The effective spring constant of the correlation of diaphragm andreadjusting spring may be made adjustable so that the natural frequencyof the drive unit can be adapted as required. A helical spring ispreferably used as the readjusting spring.

A further subject matter of the invention is a linear compressor with aworking chamber, a piston that can be moved back and forth in theworking chamber to compress a working fluid, and a drive unit of thetype described above, coupled to the piston, for driving the back andforth movement.

Further features and advantages of the invention are evident from thefollowing description of exemplary embodiments with reference to theattached figures.

FIG. 1 shows a diagrammatic section through a linear compressor;

FIG. 2 shows an elevation of a diaphragm spring for use in the linearcompressor in FIG. 1 according to the invention;

FIG. 3 shows an elevation of a second design of a diaphragm spring;

FIG. 4 shows a partially cut side view of a linear compressor with thediaphragm spring shown in FIG. 3; and

FIG. 5 shows a further design of a diaphragm spring.

The linear compressor shown in FIG. 1 for a refrigerating devicecomprises a compressor chamber 1, which is delimited by a moving piston2 on the one hand and a cylinder 3 on the other, joined together by apipe section 4 and a cover 5. Cover 5, not shown, incorporates in anintrinsically known manner a suction connection, a pressure connectionand valves which allow refrigerant to flow into the compressor chamberonly via the suction connection and discharge only via the pressureconnection.

Pipe section 4 is surrounded concentrically by a second pipe section 6and is connected to it by a radial flange 7. The circumference of adiaphragm spring 8 is fastened to the end of pipe section 6 facing awayfrom flange 7. An oscillating body 9, which is composed of a piston rod10, to which piston 2 is articulated, a flange 11 fastened to piston rod10 and a permanent magnet 12, which is fastened to flange 11 andprojects into the interval between pipe sections 6, 4, is fitted in thecentre of diaphragm spring 8. Electromagnets, also accommodated in theinterval, for exerting a force in the direction of piston rod 10 onpermanent magnets 12, are omitted in the figure.

FIG. 2 shows an elevation of diaphragm spring 8. It comprises aperipheral outer ring 13 and mirror image limbs 14 which are arranged inpairs and run spirally inwards from ring 13, which limbs are connectedto each other at their ends facing away from ring 13. A central opening15 is provided for screwing piston rod 10.

Diaphragm spring 8 consists of spring steel or another elasticallydeformable, but essentially non-expandable material. The central regionof diaphragm spring 8 can be elastically deflected with little force ina direction perpendicular to the plane shown in FIG. 2, the deflectioncausing the curvature of limbs 14 to be reduced slightly in elevationand the central region to be rotated slightly in the anticlockwisedirection. The resistance of diaphragm spring 8 against a displacementof the central region in the plane shown in FIG. 2 is much greater thanthe resistance against a deflection perpendicular to this plane, so thatthe end of piston rod 10 fastened to opening 15 of diaphragm spring 8 isreliably guided so that it moves in a linear direction.

A second design of diaphragm spring 8 is shown in FIG. 3 in elevation.This design also has a closed outer ring 13. Here this ring isrectangular in shape, but this is insignificant as far as the functionof the diaphragm spring is concerned. Four limbs 14 extend from thecorners of frame 13 towards central region 16, each of them being formedfrom three rectilinear sections 17 and two curved sections 18, 19connecting sections 17. The two sections 18, 19 of each limb 14 are eachcurved in opposite directions. Four bores 20 for fastening the diaphragmspring are located in the corners of frame 13.

When central region 16 is deflected, this results in slight upwardbending of curved sections 18, 19. Because of the opposite directions ofcurvature of the two sections 18, 19 of each limb, the upward bendinggives rise to opposing torques, so that the torque exerted by eachindividual limb 14 on central region 16 is small. Moreover, the torquesof adjacent limbs 14 are mutually compensating because each of them isthe mirror image of the other and the torques exerted by them aretherefore inversely the same. Central area 16, and consequently also apiston rod 10 fastened to it, are therefore guided exactly linearly andfree from distortion.

FIG. 4 shows a partially cut side view of a linear compressor in whichdiaphragm springs 8 of the type shown in FIG. 3 are used. The compressorhas a frame with a central chamber 21, in which openings are formed intwo opposing walls, here denoted as ceiling 22 and floor 23 withreference to the representation in the figure, for the purpose of clearillustration, through which openings a rod-shaped oscillating mass 24extends with a certain clearance. The chamber is provided to accommodateelectromagnets, not shown, for driving a back and forth movement of apermanent magnet inserted in the oscillating mass.

The ends of oscillating mass 24 are fastened to central regions 16 oftwo diaphragm springs 8 of the shape shown in FIG. 3 by means of screwsor rivets 25. Frame 13 of each diaphragm spring 8 rests in turn onbridges 26 projecting from ceiling 22 and floor 23 of central chamber221. The height of bridges 26 establishes the maximum stroke of movementof the oscillating mass 24; if this maximum stroke is exceeded, centralregions 16 of diaphragm spring 8 strike against ceiling 22 and floor 23.

Diagram springs 8 are retained on bridges 26 by screws or rivets 27,each of which intersect a foot piece 28 of an upper and lower yoke 29,30 and one of bores 19 in the corners of frame 13, and engage in centralchamber 21.

Lower yoke 30 supports two helical springs 31, each of which ispositioned so that free head piece 32 of these springs each touch curvedsections 18 of two limbs 14, as also denoted as a dash-dot outline inFIG. 3, when they are deflected downwards and therefore resist adownward deflection of oscillating mass 24. Corresponding helicalsprings 31, which touch curved sections 18 of limbs of upper diaphragmspring 8 and counteract an upward deflection of the oscillating mass,are provided on upper yoke 29.

Upper yoke 29 also supports a cylinder 33 in which a piston connected tooscillating mass 24 by means of a piston rod 10, not shown in thefigure, is able to move back and forth. Since oscillating mass 24 isguided exactly linearly by the two diaphragm springs 8, piston rod 12,and with it the piston supported by it, cannot deviate transversely tothe direction of movement and grinding of the piston against the innerwall of cylinder 33 can be avoided.

When oscillating mass 24 is located at one of the points of inversion onits trajectory, its entire kinetic energy is stored in the diaphragmsprings 8 and the helical springs 31 in the form of deformation energy,the distribution of the energy among the spring types depending on theirrespective spring constants. The diaphragm springs may therefore be madevery thin and easily deformable so that no material fatigue occurs evenduring protracted operation. For the energy which the diaphragm springsare unable to store due to insufficient stiffness may be absorbed bysuitably dimensioned helical springs 31.

Moreover, compressors with different throughputs can be achieved withthe same model of diaphragm spring if the diaphragm springs are eachcombined with helical springs with different spring constants, resultingin different natural frequencies of the oscillating system.

It is also conceivable to render the natural frequency of a drive unitadjustable by mounting helical springs 31 displaceably on yokes 29, 30.The closer the region of limbs 14 touched by head pieces 32 of helicalsprings 31 is to central region 16 of diaphragm springs 8, the stifferwill be the entire system, consisting of the diaphragm spring andhelical springs, and the higher will be the natural frequency of theresultant drive unit.

In the extreme case it is possible to replace the two helical springs 31of each yoke 29, 30 by a single helical spring which touches centralregion 16 directly.

FIG. 5 shows a modification of diaphragm spring 8 from FIG. 3, which canbe used in its stead in the compressor shown in FIG. 4. In the case ofthe diaphragm spring shown in FIG. 5, outer frame 13 is omitted andinstead only the three right and two left limbs 14 are connected attheir ends facing away from central region 16 by a material strip 34.The mode of operation is no different to that of the diaphragm springshown in FIG. 3.

1-12. (canceled)
 13. A drive unit for a linear compressor comprising aframe and an oscillating body mounted in the frame by means of at leastone diaphragm spring and moving back and forth, the diaphragm springhaving a plurality of limbs engaging the frame with a first end and theoscillating body with a second end, and a curved path between the twoends, wherein two of the limbs have inversely curved sections.
 14. Thedrive unit according to claim 13, wherein each limb has two sectionscurved in different directions.
 15. The drive unit according to claim13, further comprising a second diaphragm spring, the first and seconddiaphragm springs engaging on regions of the oscillating body that arespaced in the direction of the oscillating movement.
 16. The drive unitaccording to claim 13, wherein the limbs of the same diaphragm springare integrally joined together at their ends engaging on the oscillatingbody.
 17. The drive unit according to claim 13, wherein the limbs of thesame diaphragm spring are integrally joined together at their endsengaging on the frame.
 18. The drive unit according to claim 17, whereinthe ends engaging on the frame are connected by a frame integral withthe limbs.
 19. The drive unit according to claim 13, further comprisinga readjusting spring assigned to each limb and counteracting deformationof the limb.
 20. The drive unit according to claim 19, wherein thestiffness of the diaphragm spring is lower in the direction ofdeformation than that of the readjusting spring.
 21. The drive unitaccording to claim 19, wherein an effective spring constant of thecombination of diaphragm spring and readjusting spring is adjustable.22. The drive unit according to claim 19, wherein the readjusting springincludes a helical spring.
 23. The drive unit according to claim 13,wherein the mass of the oscillating body is greater than the mass of allthe springs.
 24. A linear compressor comprising: a working chamber; apiston being movable back and forth in the working chamber forcompressing a working fluid; and a drive unit coupled to the piston fordriving the back and forth movement and comprising a frame and anoscillating body mounted in the frame by means of at least one diaphragmspring and moving back and forth, the diaphragm spring having aplurality of limbs engaging the frame with a first end and theoscillating body with a second end, and a curved path between the twoends, wherein two of the limbs have inversely curved sections.